Heave motion compensation

ABSTRACT

A heave motion compensator ( 30, 40, 70, 100, 140, 300 ) for compensating heave motions comprises a cylinder ( 41 ) and a piston ( 44 ) delimiting a variable volume fluid chamber ( 49 ) in said cylinder ( 41 ), wherein said piston ( 44 ) can oscillate within said cylinder ( 41 ), said piston ( 44 ) being provided with a seal ( 48 ) frictionally engaging said cylinder. The compensator further includes a motor ( 65 ) that causes said seal ( 48 ) to revolve relative to said cylinder ( 41 ) so as to obtain a dynamic friction regime between the seal ( 48 ) and the cylinder ( 41 ). In a possible embodiment the motor ( 65 ) is arranged to rotate said piston ( 44 ), and the seal ( 48 ) is mounted on said piston ( 44 ) so as to rotate along with said piston ( 44 ).

The present invention relates to the field of heave motion compensation. In particular the invention relates to heave motion compensators, which can be included in a heave compensation system.

Heave motion compensation is used in many activities wherein the heave motion of the vessel—mainly induced by waves—is likely to impair said activities. For example when performing a drilling operation from a floating drill rig, the heave of the rig is compensated for in order to obtain a reduced variation of the weight on drill bit (the downward force on the drill bit).

Heave compensation systems often are employed in vessel-mounted load handling systems, such as cable-suspended load handling systems (e.g. in cranes). In a cable-suspended load handling system a load is generally suspended from a cable and commonly a winch is provided to pay out or take up the cable.

Such cable-suspended load handling systems are for instance employed to lower a load into the water (e.g. diving equipment), retrieving a load from the water (e.g. in salvage operations), placement of a load on the seabed (e.g. a template) or onto a subsea installation (BOP, wellhead equipment), or placement/lowering a load into a wellbore (well intervention equipment, logging tools, etc).

Also heave compensation is, for example, employed in load handling systems on vessels used for construction and/or demolition of offshore structures (placement of superstructure on fixed rig).

Further examples of load handling systems with heave compensation are subsea pipelaying systems, and systems for coring or scientific subsea operations.

In U.S. Pat. No. 6,595,494 (Huisman Special Lifting Equipment) heave compensation systems are disclosed. Herein two compensators are arranged in a passive heave compensation system. Each compensator is embodied as a piston and cylinder assembly having a variable volume hydraulic chamber in said cylinder. In this prior art document the variable volume hydraulic chamber is connected to a hydraulic chamber of an accumulator, said interconnected chambers being filled with hydraulic fluid. The accumulator has a freely slideable separating piston positioned between said hydraulic chamber and a gas chamber which is in turn connected to a bank of pressurised gas reservoirs. As is known in the art the pressure of the gas can be controlled, e.g. set at a desired level, by a gas pressure controller and in said manner the pressure within the variable volume hydraulic chamber of the compensators can be controlled.

In this prior art document the heave motion compensation is included in a cable-suspended load handling system, and the piston of the compensators each support a movable or “flying” sheave guiding the cable in such a manner that heave motion of the vessel then causes the piston to oscillate within the cylinder of the compensator. Hereby the path of the cable from which a load is suspended is lengthened and shortened thus cancelling at least a part of the heave motion.

Another known load handling system used in the offshore industry is the nodding-boom system, wherein a cable sheave is mounted on a boom, which boom is pivoted on a base. The boom is arranged to move up and down along with the heaving of the vessel. A compensator is mounted between the base and the boom in order to maintain a substantially constant cable tension.

In many known applications heave compensation is (also) obtained with an active heave compensation system, wherein the oscillations of the piston relative to the cylinder of the active compensator are governed by a unit supplying pressurised fluid (gas or hydraulic liquid) in a controlled manner to one or more variable volume chambers within the compensator based upon one or more input signals obtained by one or more suitable sensors (for example vertical vessel motion sensor (e.g. acceleration sensor), (cable) force sensor, compensator piston position sensor, etc).

It is also well known to combine passive and active heave compensation systems.

For example in offshore drilling operations it has been found that presently available heave compensation systems are unsatisfactory. For instance the heave motion at the lower end of a drill string still renders it impossible to use sophisticated drilling tools, e.g. directional drilling tools, taking into account the weather window wherein such drilling should be economically feasible.

In general the available heave compensation systems do not provide a satisfactory “behaviour” as to the finally obtained compensation of the heave. This for instance limits the speed of operations, the permissible weather window to carry out offshore operations, the size and/or weight of load that can be handled, imposes unwanted constraints on subsea equipment, etc.

It is an object of the present invention to provide an improved heave motion compensation system, in particular an improved heave motion compensator for such a system.

The present invention provides a heave motion compensator comprising a cylinder and a piston delimiting a variable volume fluid chamber in said cylinder, wherein said piston can oscillate within said cylinder, said piston being provided with a seal frictionally engaging said cylinder. The compensator is characterised in that said compensator further includes a motor that causes said seal to revolve relative to said cylinder.

The present invention envisages to revolve or rotate the seal with respect to the cylinder during operation of the compensator.

The effect obtained by this relative revolving or rotary motion is that the friction between the seal and the cylinder is in the dynamic regime, even when the piston is momentarily stationary in the longitudinal direction of the cylinder. In addition the frictional force is essentially tangentially directed, even when the piston is oscillating.

In the absence of the proposed revolving motion, as in all known compensators, a stationary position of the piston within the cylinder causes the friction between the seal and the cylinder to be in the static regime. When heave motion urges said known compensator to respond—by longitudinal motion of the piston relative to the cylinder—the seal “breaks away” from its position in the cylinder. In technical terms a transition from “static friction” to “dynamic friction” occurs which is known as stick-slip. The inventors have found this transition to be detrimental to the behaviour of the heave compensation system.

By revolving or rotating the seal relative to the cylinder, the invention achieves that the “transition” explained above does not occur as the friction already is within the dynamic regime. Also the frictional force is mainly in tangential direction, thus having limited effect on longitudinal motion of the piston with respect to the cylinder.

The motor employed for establishing the revolving motion can be of any suitable design, e.g. including an electric motor, a hydraulic motor, a magnetic motor, etc. A suitable transmission assembly can be provided with the motor to drive the revolving part of the compensator.

The revolving motion may include an oscillating revolving motion, but a continuous revolving motion is preferred for constructional reasons.

The inventive concept is advantageous for passive heave compensators. In particular it is envisaged that the behaviour and efficiency thereof in relative calm wave conditions is significantly improved over prior art passive heave compensators.

The inventive concept can equally be applied to active heave compensators in order to improve the behaviour thereof. In particular it will allow a more accurate compensation of the heave motions.

When applied on a floating offshore drilling rig it is possible to obtain a significantly improved heave compensation, e.g. resulting in less variation of the weight on drill bit.

The present invention can also be described as providing a heave motion compensator comprising a ram having a cylinder and a piston-piston rod assembly which is extendable and retractable relative to said cylinder, said piston-piston rod assembly frictionally engaging said cylinder, characterised in that said compensator further includes a motor adapted to cause at least said piston, preferably said piston-piston rod assembly, to rotate relative to said cylinder.

The present invention can also be described as providing a heave motion compensator comprising a piston and cylinder assembly, wherein a variable volume fluid chamber is delimited by said piston in said cylinder, said variable volume chamber being connected to an accumulator, said piston frictionally engaging said cylinder, characterised in that said compensator further includes a drive assembly adapted to provide a revolving motion of said piston relative to said cylinder when said compensator is in use.

In a preferred embodiment the motor is arranged to revolve or rotate the piston within the cylinder during operation of the heave compensator, and the seal is mounted on said piston so as to rotate along with said piston.

In another embodiment the motor is arranged to rotate the cylinder, and the piston is adapted to be mounted non-rotatable. One could envisage for instance an embodiment wherein a fluid conduit extends through the piston, which is connected to a pressurised fluid assembly external of said compensator, e.g. at a remote location.

In a further embodiment the seal is carried by a seal carrier which is mounted rotatable on said piston, and the said motor is arranged and adapted to drive said seal carrier so as to rotate it about the piston.

It can also be envisaged that the cylinder wherein the piston oscillates is an inner cylinder which is rotatably mounted within an outer cylinder, and wherein said outer cylinder is adapted to be mounted non-rotatable.

The present invention also relates to a heave compensation system including a compensator as disclosed herein.

As is known in the art, and examples are mentioned in the introduction, a heave compensation system may include a cable from which a load (drill string or other drilling tubular, object, etc) is suspended. The heave compensator may engage on a sheave (or sheave assembly) guiding said cable in order to vary the path of the cable in order to obtain heave compensation.

It is also envisaged that the compensator is placed directly between a part of the vessel subjected to heave and the load itself, for example between a travelling block in a drilling derrick, mast or other drilling structure on the one hand and the top drive unit or other suspension unit from which a drill string is suspended on the other hand.

The present invention also relates to a vessel including a heave compensation system as disclosed herein.

The present invention also relates to a vessel load handling system including a heave compensation system as disclosed herein.

The present invention also relates to a floating rig drilling system including a heave compensation system as disclosed herein. As mentioned the heave compensation system can be arranged between a heaving drilling structure on the vessel on the one hand and the drill string (or top drive supporting the drill string) on the other hand. The heave compensator could also be arranged within the drill string, e.g. at the upper end thereof.

The present invention also relates to a floating rig drill string compensator adapted for placement between a drill string or other drilling tubular and a drill string hoisting device, for instance between a top drive that allows to support and rotate the drill string and a hoisting device supporting said top drive.

The present invention also relates to a wireline logging system including a wireline cable, an associated wireline winch, and one or more instruments integrated into or fastened onto the wireline. The wireline with instruments is to be conveyed into a wellbore, said system further including a heave compensation system as disclosed herein. Wireline logging in general is the process by which oil or gas wells are surveyed to determine their geological, petrophysical or geophysical properties using electronic measuring instruments conveyed into the wellbore by means of a (armoured steel) cable, known as a wireline cable.

The present invention also relates to a floating drilling vessel including a heave compensated drill floor, wherein the drill floor is mobile relative to the vessel to compensate for heave motion of the vessel, wherein a heave compensation system as disclosed herein is arranged between the vessel and the drill floor.

The present invention also relates to a method for heave motion compensation, in particular heave motion compensated load handling on a vessel, wherein use is made of a heave motion compensator as disclosed herein, and wherein said seal is made to revolve relative to the cylinder of the heave compensator such that said friction between the seal and the cylinder is in the dynamic friction regime.

An alternative approach to obtain improved heave motion compensator behaviour, in particular to overcome the stick-slip of prior art compensators, is to provide design the piston seal as a hydrostatic bearing, wherein a narrow gap is maintained between the piston seal and the cylinder and wherein a pressurised fluid is supplied through the piston to the gap so as to create a hydrostatic bearing. The piston may then be provided with one or more annular pockets, recessed with respect to the outer perimeter of the piston, and one or more fluid channels within the piston (possibly extending through the piston rod) connected to a source of pressurised fluid so that a continuous flow of fluid can be provided to maintain the spacing between the piston and the cylinder. As such an essentially “frictionless” compensator is obtained, in particular when a further hydrostatic bearing is provided between the piston rod and an end cover on the cylinder.

It will be appreciated that this hydrostatic bearing design in a compensator does not require the revolving motion discussed herein. A drawback of the hydrostatic bearing design resides in the need to compensate flow the loss of pressurised fluid in the bearing. In practice a bank of one or more gas, preferably air, reservoirs and a compressor will be provided to feed the bearing.

The compensators according to the invention will now be explained in more detail referring to examples shown in the drawings. In the drawings:

FIG. 1 shows schematically a part of an floating vessel provided with a compensator according to the invention,

FIG. 2 shows schematically in cross-section a first example of a heave compensator and compensation system according to the invention,

FIG. 3 shows schematically in cross-section a second example of a heave compensator according to the invention,

FIG. 4 shows schematically in cross-section a third example of a heave compensator according to the invention,

FIG. 5 shows schematically a part of an floating vessel provided with a compensator according to the invention,

FIG. 6 shows schematically a part of an floating drilling rig system provided with compensators according to the invention,

FIG. 7 shows schematically a floating vessel cable-suspended load handling system provided with a compensator according to the invention,

FIG. 8 shows schematically a floating vessel cable-suspended load handling system provided with a compensator according to the invention, and

FIG. 9 shows schematically in cross-section a further example of a heave compensator and compensation system according to the invention.

FIG. 1 shows a vessel 1, here for illustrative purposes an offshore drilling or well intervention vessel, having a drilling structure 2 thereon from which a drill string 3 or other drilling tubular (e.g. a riser) is suspended into the sea. In this example the vessel 1 is provided with a moonpool 4 and the drilling structure is embodied as a mast which is arranged adjacent said moonpool 4.

The vessel is equipped with a hoisting device. In this example a winch 6 is provided for paying out and taking up a cable 7. This cable 7 is guided over sheave assemblies 8, 8 a, 8 b, 9 in the structure 2 and a sheave assembly 10 on a travelling block 11. The travelling block 11 moveable up and down with respect to the structure 2, here guided along on or more vertical guide rails 12 mounted on the mast 2.

For performing drilling operations here a top drive unit 20 is provided, which can support the drill string suspended therefrom as well as impart rotary motion to the drill string in order to rotate the drill bit at the lower end of the drill string (not shown).

In this example the top drive unit 20 is also guided vertically with respect to the structure 2, mainly in order to counter the torque exerted by the top drive unit. Here the top drive unit 20 is received in a trolley 21 which is guided along one or more vertical guide rails 12 on the mast 2.

The top drive unit 20 may include a hydraulic motor to impart the torque to the drill string and a drill string clamp device to clamp and support the suspended drill string.

Between the top drive unit 20 and the travelling block 11 a floating rig drill string compensator is placed that allows to compensate for heave of the vessel 1. The compensator 30 here is a passive heave compensator.

Basically the compensator 30 allows the travelling block 30 to move up and down as a result a waves (possibly including roll and/or pitch of the vessel) whereas the drill string 3 should remain unaffected by said heave motion. This in order to obtain a controlled weight on drill bit with a variation which is as low as possible.

In FIG. 1, which merely is an example, also a further heave compensator 22 is shown, carrying flying sheave assembly 9. It is noted that in this FIG. 1 said compensator 22 is provided when the hoisting device is used for other purposes than drilling, e.g. when the device is used to raise or lower loads through the moonpool other than the drill string or the like. It will be appreciated that said heave compensator can be an active heave compensator.

Referring to FIGS. 2-4 a number of examples of such a heave motion compensator will be discussed in more detail. Each of them could be arranged at the location of compensator 30 in FIG. 1.

In FIG. 2 a heave compensation system is shown (not to scale) including a heave motion compensator 40. The compensator 40 includes a cylinder 41 having end covers 42, 43. A piston 44 arranged on a piston rod 45 is placed within the cylinder 41 and can oscillate in longitudinal direction within said cylinder 41. The piston rod 45 extends through an opening with surrounding seal 46 in the end cover 42.

The piston 44 is provided with a seal 48 extending around the outer perimeter of the piston 44, which seal closes the gap between the piston 44 and the cylinder 41. The seal 48 frictionally engages the inner wall of the cylinder 41.

Between the piston 44 and the end cover 42 a variable volume fluid chamber 49 is delimited in the cylinder 41, the volume depending on the axial position of the piston 44.

At the other side of the piston 44 an ambient pressure chamber 50 is formed within the cylinder 41, said chamber 50 being in communication with the atmosphere, e.g. via opening 51.

The FIG. 2 shows that the chamber 49 is connected via a duct 52 to a pressurised fluid assembly here including an accumulator 53 having a variable volume hydraulic chamber 54 and a variable volume gas chamber 55 and a separation there between (here a free sliding piston 56). The chambers 49, 54 and the duct 52 are filled with hydraulic liquid, whereas the chamber 55 is gas filled.

The chamber 55 is in turn connected to a bank of one or more gas reservoirs 57, which preferably have a large volume of gas therein. The gas can in practice be air or nitrogen.

A gas pressure controller 58 is provided to set and adjust if desired the gas pressure within chamber 55 and in this manner set the pressure within the chamber 49. This pressure creates an inward force on the piston/piston rod-assembly.

It is noted that the pressurised fluid assembly (or parts thereof) can be located remote from the compensator 40, but could also be arranged on a common carrier, e.g. mounted on the travelling block 11 or suspended therefrom.

FIG. 1 further shows that the compensator 40 is provided at its lower end with a connector 60 for connecting the compensator to the top drive unit 20. The connector 60 is here designed such that during operation of the drilling system the cylinder 41 does not rotate about its axis.

The piston rod 45 is attached to a journaled connector 61, which connector 61 connects the compensator to the travelling block 11. The connector 61 includes a rotary bearing assembly 62 between connector part 63 to be attached to the travelling block 11 and the piston rod 45. The rotary bearing assembly 62 allows the piston rod (and the piston arranged thereon) to rotate about its longitudinal axis while the connector part 63 remains non-rotating.

Mounted on said non-rotating connector part 63 is a motor 65, here an electric or hydraulic motor having a rotatable output shaft 66. A transmission 67 between the motor 65 and the piston rod 45 allows to impart to the piston rod a revolving or rotary motion. Here the transmission is designed as a gear 68 on the shaft meshing with a gear 69 fixed on the piston rod 45. It will be appreciated that many other motor and transmission arrangements may be designed to achieve the revolving motion of the piston rod 45.

By rotation of the piston rod 45 also the piston 44 is rotated and thus the seal 48 mounted on said piston 45. It is proposed that during operation of the heave compensation system the piston 44 and thus the seal 48 is kept revolving continuously.

The effect caused by said revolving motion is that the friction between the seal 48 and the cylinder 41 (and also between the end cover seal 46 and the piston rod 45) is a dynamic friction, even when the piston is stationary in the longitudinal direction of the cylinder 41. This prevents the “jerky” transition from static friction to dynamic friction as is commonly experienced in prior art passive heave compensation systems.

In addition the frictional force between the seals 46, 48 and the cylinder 41 with end cover 42 is mainly in tangential direction. This means that the frictional force vector in longitudinal direction, effectively counteracting the motion of the piston, is very small.

Both aspects mentioned above (dynamic friction, mainly in tangential direction) result in a significantly improve the behaviour of the compensator 40, in particular when compensating rather small heave motions. Such small heave motions can now be accurately compensated for as the piston 44 seems to slide “frictionless” within the cylinder 41.

It is noted that the motor 65 here provides a continuous rotary motion of the piston 44. It can also be envisaged that the motor 65 provides an oscillating rotary motion of the piston 44, e.g. a back and forth motion over less than 360 degrees angle, possibly allowing for the angle to vary from time to time to avoid uneven or local wear of the seal and/or cylinder.

FIG. 3 shows a heave motion compensator 70 having a cylinder 71, a piston 72 and piston rod 73 with end fitting 73 a protruding through an end cover 74 of said cylinder 71. A variable volume fluid chamber 76 is delimited in said cylinder 71 between the piston 72 and the end cover 74. A fluid duct 75 connects the chamber 76 to a suitable pressurised fluid assembly, e.g. via an accumulator 54,55 to a bank of one or more gas reservoirs 57 containing pressurised gas similar to FIG. 2.

The compensator 70 includes a seal 78 between the piston 72 and the interior wall of the cylinder 71. Here the seal 78 is carried by a seal carrier 79 which is mounted rotatable on said piston 72. A bearing assembly 80 (and a seal not shown) is provided here between the piston 72 and seal carrier 79. This arrangement allows for rotation of the seal carrier 79 while the piston and piston rod assembly itself is non-rotatable.

A motor 81 is mounted here at the side of the piston-piston rod assembly remote from the chamber 76. The motor 81 drives said seal carrier 79, here as a shaft of the motor carries a pinion 82 meshing with a ring gear 83 on the carrier 79. The skilled person will appreciate that other motor and transmission arrangements may be employed to cause the rotary motion of the seal carrier 79.

In the example shown the motor 81 is a hydraulic motor, flexible hydraulic lines 84 being connected to said motor 81.

It will be appreciated that again the effect is achieved that the seal 78 can be kept in rotation or rotational oscillation continuously as the system is in operation.

FIG. 4 shows a heave motion compensator 100 having a inner cylinder 101 which is rotatably mounted within an outer cylinder 102, here supported by bearings 103, 104. The outer cylinder 102 is adapted to be mounted in a non-rotating manner, e.g. connected via connector 106 to a non-rotating element (such as the body of a top drive unit).

The compensator 100 includes a piston 107 and piston rod 108 with end fitting 108 a, which protrudes through an end cover 110 of the compensator. A seal 109 is mounted on the piston 107. A variable volume fluid chamber 111 is present connectable to a pressurised fluid source via duct 112. The duct 112 is provided in a non-rotatable part of the compensator, e.g. the cylinder 102 or the end cover 110 to facilitate said connection.

A motor 115 is arranged such that it imparts revolving motion to the inner cylinder 101. Here the motor 115 is fixed on the outer cylinder 102, here on the end cover 116, in a stationary position. The motor 115 here is provided with a rotatable pinion 117 meshing with a ring gear 118 on the inner cylinder 101.

As will now be understood the compensator 100 can be operated such that the motor 115 drives the inner cylinder 101 and thus a relative rotary motion between the seal 109 and the inner cylinder 101.

A seal 120 is provided between the inner cylinder 101 and the outer cylinder 102 at a suitable location.

FIG. 5 shows an alternative of the load hoisting system of FIG. 1, wherein the compensator 40 is positioned between the hoisting structure, here mast 2, and a flying sheave assembly 9. In this example it is envisaged that the cable 7 supports the top drive 20 directly, without an interpositioned compensator as in FIG. 1.

FIG. 6 shows a part of a floating drill rig system, having a travelling block 130, which maybe suspended from a cable or other raising/lowering device, and a top drive 135 suspended therefrom. Between the travelling block 130 and the top drive 135 are two compensators 140, arranged in an inverted V, their lower ends connected to the travelling block 110 and their upper ends both to a common connector 130 from which the top drive 120 is hanged. This arrangement of two compensators 140 according to the invention in a V-arrangement, basically symmetrical to the path of the member supported by the compensators, provides a practically attractive solution. The compensators 140 each have a piston 141 held non rotatable, whereas the cylinder 142 is rotated by an associated motor 143. A bearing is arranged between the cylinder 142 and the connector 144, which is connected to the travelling block 130.

In general it can be considered advantageous when an inventive compensator is arranged at an angle with respect to the path of the member supported thereby.

FIG. 7 shows application of a compensator 140 in a vessel mounted crane 150. The crane 150 has a boom 151, a topping cable 152 and winch 153, and a load carrying cable 154, and associated winch 155. The compensator 140 here is arranged to movably support a sheave assembly 156, here arranged on the boom 151, along which the cable 154 is guided, said cable supporting a crane hook 157

FIG. 8 illustrates the nodding boom alternative. A part of a vessel 200 is shown having a crane arm 210, e.g. embodied as an A-frame, pivotable with respect to the vessel about a horizontal axis 211. A winch 212, cable 214 guided over a sheave assembly 213 on the (end of the) arm 210 supports a load 215 which is to be raised and/or lowered by the crane (e.g. for placement of the load onto the seabed 216). To obtain heave compensation a compensator 40 is arranged between the vessel and the arm 210.

FIG. 9 shows an alternative improved heave motion compensator 300 wherein the stick-slip effect is avoided in a different manner. The compensator 300 has a cylinder 301 with end covers 302, 303. A piston 304 arranged on a piston rod 305 is placed within the cylinder 301 and can oscillate in longitudinal direction within said cylinder 301. The piston rod 305 extends through an opening with surrounding seal 306 in the end cover 302.

Between the piston 304 and the end cover 302 a variable volume fluid chamber 309 is delimited in the cylinder 301, the volume depending on the axial position of the piston 304.

At the other side of the piston 304 chamber 310 (possible at atmospheric pressure) is in this example formed within the cylinder 301.

The FIG. 9 shows that the chamber 309 is connected via a duct 315 to a pressurised fluid assembly here including an accumulator 316 having a variable volume hydraulic chamber 317 and a variable volume gas chamber 318 and a separation there between (here a free sliding piston 319). The chambers 309, 317 and the duct 315 are filled with hydraulic liquid, whereas the chamber 318 is gas filled.

The chamber 318 is in turn connected to a bank of one or more gas reservoirs 320, which preferably have a large volume of gas therein. The gas can in practice be air or nitrogen.

A gas pressure controller 322 is provided to set and adjust if desired the gas pressure within chamber 318 and in this manner set the pressure within the chamber 309. This pressure creates an inward force on the piston/piston rod-assembly.

The piston 304 has a hydrostatic bearing which supports (basically centers) the piston with respect to the cylinder, wherein a narrow annular gap is maintained between the piston 304 and the cylinder 301. A pressurised fluid, here hydraulic liquid, is supplied from a suitable source 340 through the piston (via conduit 325) to the hydrostatic bearing on the piston. The piston 304 here by way of example is provided with one or more annular pockets, here a tapering pocket 330, which is recessed with respect to the outer perimeter of the piston. The conduit 325 extends within the piston and through the piston rod. A continuous flow of liquid is provided to the hydrostatic bearing in order to maintain the spacing between the piston and the cylinder. As such an essentially “frictionless” compensator is obtained.

The source 340 of pressurised fluid for the hydrostatic bearing can include an gas (air) source or a hydraulic liquid source. It can be envisaged that hydraulic liquid leaking from the hydrostatic bearing is collected in the compensator 300 (preferably within the chamber 310) and returned to the source 340, so as to create a “circulation circuit” for said liquid. A pressure controller can be provided to control the pressure of the fluid supplied to the hydrostatic bearing on the piston.

In an embodiment not shown in the drawings, generally as an alternative version of the FIG. 3 embodiment, it is envisaged that the seal carrier is mounted on the piston so as to allow for a longitudinal oscillation of the seal carrier with respect to the piston independent from the longitudinal oscillations of the piston itself. This also allows to create a dynamic friction regime between the seal and the cylinder and thus avoids the transition between static friction and dynamic friction when the piston starts to move longitudinally. It will be appreciated that the same can be realised with an inner cylinder oscillation longitudinally within an outer cylinder, generally as an alternative to the FIG. 4 embodiment.

In regard of the above the invention can also be understood so as to provide a heave motion compensator comprising a cylinder and a piston delimiting a variable volume fluid chamber in said cylinder, wherein said piston can oscillate within said cylinder, said piston being provided with a seal frictionally engaging said cylinder, characterised in that said compensator further includes a motor that imparts a relative motion between said seal and said cylinder independent from said piston oscillation.

Referring by way of example to the FIG. 2 embodiment of the compensator it will be appreciate that the same rotary drive system for the piston and piston rod can be integrated in a compensator which has two variable volume fluid chambers within said cylinder and separated by the piston. Such compensators are commonly used in active heave compensation systems, wherein further provision is made for a pressurised fluid assembly that allows to selective supply and discharge of fluid to and from said variable volume fluid chambers so as to cause controlled oscillation of said piston within said cylinder to obtain heave compensation. As mentioned in the introduction such active systems commonly include one or more sensors providing input signals for a control unit, which governs the fluid supply and thus the position of the piston. Here too the motion of the seal relative to the cylinder independent from the piston oscillations is advantageous for the behaviour of the compensation system. 

1. Heave motion compensator comprising a cylinder and a piston delimiting a variable volume fluid chamber in said cylinder, wherein said piston can oscillate within said cylinder, said piston being provided with a seal frictionally engaging said cylinder, characterised in that said compensator further includes a motor that causes said seal to revolve relative to said cylinder.
 2. Compensator according to claim 1, wherein said motor is arranged to rotate said piston, and wherein said seal is mounted on said piston so as to rotate along with said piston.
 3. Compensator according to claim 1, wherein said motor is arranged to rotate said cylinder, and wherein said piston is adapted to be mounted non-rotatable.
 4. Compensator according to claim 1, wherein said seal is carried by a seal carrier which is mounted rotatable on said piston, and wherein said motor drives said seal carrier so as to rotate about the piston.
 5. Compensator according to claim 3, wherein the cylinder is an inner cylinder which is rotatably mounted within an outer cylinder, and wherein said outer cylinder is adapted to be mounted non-rotatable.
 6. Heave compensation system including a compensator according to claim
 1. 7. Heave compensation system according to claim 6, further including a pressurised fluid assembly interconnected to said variable volume fluid chamber and adapted to provide a controlled fluid pressure therein, possibly a constant fluid pressure independent of piston oscillation.
 8. Heave compensation system according to claim 7, wherein said pressured fluid assembly includes one or more gas reservoir storing pressurised gas therein.
 9. Heave compensation system according to claim 8, wherein said variable volume fluid chamber of said compensator is a variable volume gas chamber interconnected to said one or more gas reservoirs.
 10. Heave compensation system according to claim 7, wherein said pressurised fluid assembly includes an accumulator having a variable volume hydraulic chamber and a variable volume gas chamber and a separation there between, said variable volume fluid chamber being interconnected to said variable volume hydraulic chamber and filled with hydraulic liquid, said variable volume gas chamber being filled with pressurised gas.
 11. Heave compensation system including a compensator according to claim 1, further including a pressurised fluid assembly interconnected to said variable volume fluid chamber and adapted to provide a controlled fluid pressure therein possibly a constant fluid pressure independent of piston oscillation, wherein said pressurised fluid assembly includes an accumulator having a variable volume hydraulic chamber and a variable volume gas chamber and a separation there between, said variable volume fluid chamber being interconnected to said variable volume hydraulic chamber and filled with hydraulic liquid, said variable volume gas chamber being filled with pressurised gas, and wherein said variable volume gas chamber of the accumulator is connected to said one or more gas reservoirs.
 12. Heave compensation system according to claim 6, wherein said compensator has two variable volume fluid chambers within said cylinder and separated by said piston, a pressurised fluid assembly being provided allowing selective supply and discharge of fluid to and from said variable volume fluid chambers so as to cause controlled oscillation of said piston within said cylinder to obtain heave compensation.
 13. A vessel including a heave compensation system according to claim
 1. 14. A vessel load handling system including a heave compensation system according to claim
 1. 15. A floating rig drilling system including a heave compensation system according to claim
 1. 16. A floating rig drill string compensator according to claim 1 and adapted for placement between a drill string or other drilling tubular and a drill string hoisting device.
 17. A wireline logging system including a wireline cable, an associated wireline winch, and one or more instruments to be conveyed into a wellbore, said system further including a heave compensation system according to claim
 1. 18. A floating drilling vessel including a heave compensated drill floor, wherein the drill floor is mobile relative to the vessel to compensate for heave motion of the vessel, wherein a heave compensation system according to claim 1 is arranged between the vessel and the drill floor.
 19. Method for heave motion compensation, in particular heave motion compensated load handling on a vessel, wherein use is made of a motion compensator according to claim 1, and wherein said seal is made to revolve relative to the cylinder of the heave compensator such that said friction between the seal and the cylinder is in the dynamic friction regime.
 20. Heave motion compensator comprising a cylinder and a piston delimiting a variable volume fluid chamber in said cylinder, wherein said piston can oscillate within said cylinder, said piston being provided with a seal frictionally engaging said cylinder, characterised in that said compensator further includes a motor that imparts a relative motion between said seal and said cylinder independent from said piston oscillation.
 21. Heave motion compensator according to claim 20, wherein compensator is adapted so that a relative longitudinal oscillation of the seal with respect to the cylinder is caused by the motor independent of the longitudinal oscillation of the piston within the cylinder.
 22. Heave motion compensator according to claim 21, wherein the seal is mounted in a seal carrier, which seal carrier is mounted on the piston so as to allow for a longitudinal oscillation of the seal carrier with respect to the piston independent from the longitudinal oscillations of the piston itself.
 23. (canceled) 